Chemical-mechanical polishing (CMP) is used for wafer surface layer planarization in an ever widening number of applications. The proliferation of CMP apparatus in the manufacturing environment has been accompanied by industry pressure to increase throughput and to reduce costs.
In the CMP process a wafer carrier urges a wafer against a rotating platen which carries a polishing pad. One way to increase throughput is to increase the pressure on the wafer. At present, such apparatus allows a pressure of about 10 pounds per square inch on the wafer. A significantly increased pressure per square inch is desirable though to obtain faster removal rates. But the commercially available apparatus is not capable of permitting such pressures without having unwanted effects on wafer uniformity.
A typical wafer carrier assembly is shown in, for example, U.S. Pat. No. 5,329,732 issued Jul. 19, 1994. The assembly comprises three subassemblies: The first subassembly is at the top of the wafer carrier and is attached along its central axis to the actuator which raises, lowers, and rotates the wafer carrier. This first assembly has an enlarged diameter lower section. The second subassembly has a top portion which has a C shaped cross section which engages the enlarged diameter lower section of the first assembly. The second (or center) subassembly is spring-loaded on the top of the enlarged diameter section and includes ball bearings which permit the second assembly to cant with respect to the first assembly.
The third (or bottom) subassembly comprises the wafer holder and is fixedly connected to the bottom of the second subassembly. Some CMP apparatus includes a wafer guard inside of a guard ring which encompasses the wafer being polished.
In all presently available CMP apparatus, all those subassemblies rotate and are driven from the top along the central axis. An increase in pressure on a wafer in such apparatus causes the wafer to pull to one side and pick up on the other side because the apparatus does not permit the pressure to be distributed evenly. It is difficult to achieve evenly distributed pressure in prior art CMP apparatus.